Previous analyses (ontogenetic, morphological, and molecular phylogenetic) of Rosaceae provide strong evidence for a single origin of epigynous flowers and fleshy fruits from an ancestor with perigynous flowers that produce dry, dehiscent fruits. Furthermore, ontogenetic evidence implies that epigyny in Rosaceae begins to develop soon after initiation of carpel primordia, and involves fusion between the dorsal portions of developing carpel primordia and the developing hypanthium. To better understand changes in ontogeny that lead to epigynous vs. perigynous flowers we've investigated expression patterns of two MADS-Box genes (AGAMOUS and PLENA) and one Yabby Family Gene (CRABS CLAW). The former are required for stamen and carpel identity, while the latter plays a role in carpel morphogenesis, specifically polarity of the developing gynoecium. We used DIG-labelled mRNA probes for in-situ hybridization analyses of various stages of Physocarpus opulifolius floral development; a species with perigynous flowers that produce dry, dehiscent fruits. This species was chosen to set a baseline for future analysis of an epigynous species (Amelanchier) that produces fleshy fruits, as well as the hypothesized basal species of the clade that includes all epigynous species, Gillenia (perigynous flowers that produce dry dehiscent fruits). The primary goal of the current research is to determine expression patterns of CRC, AG and PLE genes in Physocarpus. Consistent with results from other species, CRC is expressed in the abaxial region of developing carpels of Physocarpus, whereas AG and PLE are expressed throughout initiation and development of petals, stamens and carpels. Comparative studies of expression patterns of the genes involved in determining carpel identity will increase our understanding of how these genes work together to control and mediate the formation of Rosaceae carpels in perigynous vs epigynous flowers, modifications that ultimately led to profound changes in fruit morphology and seed dispersal.